1. Field of the Invention
The present invention relates to a method and an arrangement for drying a gas, in particular natural gas, using a glycol as well measures to purify contaminated glycol in a gas drying plant.
2. Related Art
Glycols, in particular triethylene glycol, are used in great quantities to dry gases, in particular natural gas. When natural gas is transported at high pressure, the water vapour content must not exceed a certain amount which depends on the pressure. Condensed water may cause pipeline corrosion. Gas hydrates might also form.
In order to reduce the water content of the gas, it is brought into contact with glycol as an absorbent in drying plants. For example, the gas to be dried and glycol act on each other in countercurrent in an absorber. During this process, the glycol not only absorbs the moisture from the gas but also impurities and leaves the absorber laden with water and impurities. It is then regenerated, the water from the glycol-water mixture being eliminated.
The U.S. Pat. No. 5 490 873 teaches a method in which the vapours released in the regeneration are partially condensed and then fed into a separator. Three phases are separated from one another in the separator, a gas phase, a liquid hydrocarbon phase and a water phase. The gas phase is compressed and fed to the process. The liquid hydrocarbon phase is withdrawn for use and the water phase is disposed of.
The dried glycol from the regeneration process is then returned to the absorber. During this cycle the glycol becomes more and more contaminated. The impurities include salts, particulate matter, the decomposition products of the glycol as well as organic impurities which have been absorbed from the gas to be dried. Some of the decomposition products of the glycol and the impurities absorbed from the gas to be dried are organic acids, which may lead to a fall in the pH, which in turn may cause increased plant corrosion. Furthermore, these and other organic impurities and decomposition products can lead to foam formation, which reduces the drying properties of the glycol and can lead to high glycol losses.
Therefore, in conventional plants it is necessary to replace or purify the glycol at regular intervals. The glycol removed from the drying plant is normally purified by distillation.
An object of the present invention is to develop a gas drying process which permits more economic use of the glycol used, in particular extends the life of the glycol.
According to the present invention, this object is achieved by a method for drying a gas using a glycol, wherein the gas is fed into an absorber, brought into contact with glycol in the absorber, the glycol absorbing at least part of the moisture and impurities from the gas, and then the gas is removed from the absorber. The glycol laden with water and impurities is removed from the absorber and regenerated by heating to drive the water out. At least some of the glycol removed from the absorber is purified before or after it is regenerated by mixing it with at least half the quantity of water, bringing the mixture to a temperature above its cloud point, and keeping the mixture at that temperature for a predetermined time, during which time the impurities flocculate, removing the flocculated impurities, and regenerating the glycol by heating it to drive the water out of the mixture. The regenerated glycol may then returned to the absorber. Surprisingly, it was found that a considerable proportion of the impurities in the glycol are absorbed in this way by the floccules forming above the cloud point so that subsequent separation produces sufficiently purified glycol. In particular, impurities such as oil, black sludge and other oxidation products are removed from the glycol during this process. The inventive method is a very cost-effective drying process as it integrates environmentally-friendly and simple purification of glycol.
In one embodiment, the glycol removed from the absorber is purified before its regeneration. The water added during the purification process is driven out together with the water absorbed in the absorber in one regeneration step by heating. In this mainstream process, the entire amount of glycol to be regenerated, i.e. to be freed of water, is always purified before regeneration. As in this process an appropriate amount of water always has to be added to the entire amount of glycol in circulation, a large amount of energy is required to separate the water and the glycol in one regeneration step.
In a further embodiment, part of the glycol previously regenerated is purified. The glycol is first regenerated in the conventional manner by eliminating the water. Some of the regenerated but still contaminated glycol is then passed into a purification process. The rest of the regenerated glycol is fed into the absorber.
After removal of the flocculated impurities, the glycol-water mixture is preferably mixed again and regenerated with the glycol which has been removed from the absorber and is to be regenerated. This side-stream process makes it possible to purify only part of the glycol in the absorber cycle so that the energy required in the regeneration step to drive out the water absorbed from the gas and the water mixed during the purification process can be reduced.
The glycol-water mixture is preferably filtered and/or centrifuged to remove the flocculated impurities. Apart from these fast separation methods, it is also possible to employ alternatives such as sedimentation or flocculation.
In an advantageous embodiment of the method according to the present invention, the glycol-water mixture is passed over a first anion exchanger prior to heating.
The glycol-water mixture is preferably brought to a temperature of approx. 40xc2x0 C. to 90xc2x0 C. before purification and kept at that temperature for between 2 and 30 minutes. The temperature, time required and percentage of water in the mixture depend on each other. A higher percentage of water leads to more rapid clouding or flocculation of the impurities; on the other hand a higher percentage of water increases the time and amount of energy required to drive the water out of the mixture. The contaminated glycol is preferably mixed with water, roughly in a ratio of 1:1.
In a further embodiment of the inventive method triethylene glycol (TEG) is used as a desiccant. The contaminated triethylene glycol is mixed with water in a ratio of 1:1 for the purification process and the triethylene glycol-water mixture is brought to a temperature of approx. 75xc2x0 C. to 85xc2x0 C. and kept at that temperature for between 5 and 20 minutes. The temperature is above the so-called cloud point. The mixture of water and TEG is preferably passed over an anion exchanger prior to heating.
The object of the present invention is also achieved by a purification method for cleaning glycol contaminated in a gas drying plant. Contaminated glycol laden with water and impurities is removed from the gas drying plant. The glycol removed is mixed with at least half the amount of water. The glycol-water mixture is then passed over a first anion exchanger, anionic surfactants being removed from the mixture. The cloud point is reduced below the temperature of the mixture causing flocculation of the impurities. The flocculated impurities are then removed. The purified glycol-water mixture is returned to a regeneration stage of the gas drying plant where the water is driven out. The glycol removed is preferably removed from the gas drying plant and passed to the purification step at a temperature which is high enough for the temperature of the glycol-water mixture after the removal of the anionic surfactants to be already above the cloud point without it being necessary to additionally heat the glycol-water mixture before or after it has passed through the ion exchanger. to additionally heat the glycol-water mixture before or after it has passed through the ion exchanger.
The inventive method produces excellent purification results; a considerable amount of the impurities in the glycol is absorbed by the floccules which are removed. In particular, the method reliably removes oils, black sludge and oxidation products from the glycol. The inventive method is highly efficient and environmentally-friendly.
The method according to this embodiment of the invention has a number of advantages. The method is more cost-efficient and facilitates continuous operation of a purification plant as the glycol-water mixture either does not have to be heated as much after the addition of the water or does not have to be heated at all. This is achieved by lowering the cloud point, which is based on the following mechanism: The glycol from gas drying plants is very often contaminated by high boiling fats and oils. These substances are present in the glycol in emulsion. Apart from non-ionic fatty alcohol ethoxyylates and sugar surfactants, long-chain anionic carboxylic acids (fatty acids) act as emulsifiers. These substances are dissociated, i.e. charged, in the glycol at the pH of 7 normally set by the addition of amines. Therefore, the micelles of the emulsion containing the impurities also have negative charges which means that they repel each other and counteract a breakdown of the emulsion. Once the anionic surfactants have been removed from the glycol-water mixture in the first ion exchanger, the now uncharged micelles agglomerate and can be more easily eliminated. This leads to a lowering of the cloud point, i.e. the temperature at which the impurities flocculate.
The first ion exchanger is preferably charged with anions of a strong acid, in particular with chloride ions. The ion exchanger resin gives off a metered amount of chloride ions to the glycol-water mixture, the surface-active carboxylic acid anions being absorbed from the mixture (as well as other substances). The ion exchanger can be regenerated by back-flushing with a sodium chloride solution. The release of chloride ions to the glycol-water mixture additionally increases the ion strength of the mixture, which further reduces the solubility of the micelles containing the impurities. This leads to a further reduction in the cloud point temperature at which the micellar phase separates from the solution. Under these conditions, it is no longer necessary to heat the glycol-water mixture to achieve a flocculation of the impurities.
In an advantageous embodiment of the inventive method, the purified glycol-water mixture is passed, before being returned to the regeneration stage of the gas drying plant, through a basic second ion exchanger charged with hydroxide ions, the anions fed to the mixture in the first ion exchanger being at least partially replaced by hydroxide ions. Replacement of the acid anions by hydroxide ions prevents corrosion of the drying plant by the acid anions, in particular the chloride ions. A highly basic ion exchanger is preferably used. The replacement of acid anions by hydroxide anions is also described as desalination of the glycol. The second ion exchanger can be regenerated for example with sodium hydroxide solution.
In an advantageous embodiment of the present invention, the glycol-water mixture is passed through a sand or gravel filter to remove the flocculated impurities. A multi-stage filter is preferably used, the mixture first being passed through a coarser and then a finer sand fraction. The pressure rises as the filter becomes loaded with flocculated impurities. Then the sand filters have to be regenerated by back-flushing. Part of the purified glycol-water mixture can be used for this purpose.
A further embodiment of the present invention is an arrangement for drying a gas, in particular natural gas, using a glycol, which arrangement comprises an absorber in which the gas to be dried and glycol act upon each other. The absorber is linked to a means for removing the water-laden glycol from the absorber and for feeding the glycol to a reboiler for regenerating the glycol, a means for returning the regenerated glycol to the absorber, a mixing means coupled to the means for returning the regenerated glycol to said absorber, a means for removing part of the regenerated glycol, and a means for adding a predetermined amount of water, a settling chamber coupled to said mixing means, and a separating means coupled to said settling chamber for separating flocculated impurities, the separating means being coupled to the reboiler. The arrangement according to the invention is a side-stream method that conserves energy for drying the gas.
In one embodiment the device for returning the regenerated glycol to the absorber exhibits a storage vessel, the mixing device being connected to the storage vessel, preferably via a pump. A fine filter is used as the separating device.
An anion exchanger for pre-cleaning the glycol-water mixture is preferably integrated between the mixing device and the settling chamber.
According to a further feature of the present invention, an ion exchanger can be integrated between the mixing device and the separating device, anionic surfactants being removed from the glycol-water mixture in said ion exchanger.
A further embodiment of the invention is an arrangement for purifying glycol contaminated in a gas drying plant, wherein the gas drying plant contains a regeneration stage for driving out the water from a glycol-water mixture, the arrangement comprising a mixing means, a first ion exchanger, a filter means, and a regeneration stage means. The mixing means is used for mixing the glycol removed from the gas drying plant with water to form a glycol-water mixture, the first ion exchanger for removing anionic surfactants from the glycol-water mixture, and the filter means for removing flocculated impurities. The filter means may be located downstream of the first exchanger and coupled through a line to the regeneration stage for driving the -water out of the mixture.
In a preferred embodiment, the first ion exchanger is charged with anions of a strong acid, preferably with chloride ions.
An advantageous embodiment of the invention is characterised in that a basic, second ion exchanger filled with hydroxide ions is located downstream of the filter device to replace at least part of the anions fed to the glycol-water mixture in the first ion exchanger by hydroxide anions. This prevents an additional corrosive attack of the acid anions.
The filter device can exhibit a multi-stage sand or gravel filter in which coarser and downstream finer sand fractions are arranged one after the other. An active carbon filter can also be provided in addition. Both the first and the second anion exchanger and the filter device are equipped with devices (pipes, valves and corresponding control units) which permit back-flushing for regeneration or cleaning.
A preferred embodiment of the arrangement according to the present invention is characterised in that the mixing device is connected via a line to a pump of the gas drying plant passing the glycol to an absorber, that part of the glycol leaving the pump is pressed into the mixing device. Such a link-up of the purification cycle makes it possible to dispense with an additional pump in the purification cycle. Furthermore, the temperature of the glycol present in the gas drying plant cycle at this point is high enough to make it unnecessary to additionally warm the glycol to above the cloud point.
In the following the invention is described with the aid of an embodiment shown in the drawing.